made ftp connection shared
made ftp connection shared

That avoids issues with many ftp connections opened and never closed (and that causes some ftp
servers to deny any new connections from same source).

This unfortunately also made this library non-threadsafe, but this problem can be figured out later,
if needed. Thread safety is not needed anyway in 99% of Rails apps.

Also added some debugging code around connection to be enabled by ftp debugging mode.

DamianCaruso/paperclipftp